German Offenlegungsschrift 23 04 334 and corresponding U.S. Pat. No. 3,828,813 disclose a control apparatus, for achieving load-independent regulation of the flow in a hydraulic power system or the like, in which a pressure difference balance (hereinafter referred to as pressure balance) device serves to maintain a constant pressure difference between a pump pressure line and an associated consumer. The pressure balance is primarily comprised of a piston that is movable within a cylindrical chamber. A connection between the chamber and the pump pressure line subject one side of the piston to the pump pressure. Another connection subjects the other side of the piston to the system load or consumer pressure. Additionally, the other side of the piston is engaged by and subjected to the biasing force of a compression spring, which is the so-called balance spring. The piston takes a balanced or equilibrium position which is determined and defined by the pump pressure on the one hand, and by the sum of the load pressure and the spring force on the other hand. The cylindrical chamber of the pressure balance is connected to the fluid tank of the hydraulic system by an opening which is opened or closed to a greater or lesser extent by a control edge of the piston, and which when open permits the passage into the tank of some more or less large portion of the flow of hydraulic fluid delivered by the pump.
European Pat. No. 15492 and corresponding U.S. Pat. No. 4,303,091 disclose a pressure balance whose operation is further assisted by a hydraulic support assembly having a hydraulically biased differential piston. The piston is adjustable between an idling position and a working position, and supports an abutment of the balance spring of the pressure balance. An adjustable stop element extending to the exterior of the hydraulic support assembly permits adjustment of its spring supporting action.
An object of the present invention is the provision of a proportional action valve apparatus that is so constructed as to permit changes to be made in its operating characteristics, during operation of the apparatus and by varying the working position of the spring abutment of the pressure balance, such that at desired times the consumer can be hydraulically driven independently of the load at either a constant speed and finely adjusted pressure differential, or at its highest possible speed and in accordance with some other different operating characteristic. As used herein, the term "characteristic" means the dependent relationship, which may be represented by a diagram, between the volume flow of fluid through the throttle valve of the apparatus and the control signal or force that actuates the throttle valve.